Parental care liberates juvenile growth: a common-garden test of the evolutionary benefits of care

Effects on juvenile growth have long been considered an important benefit of parental care, but they have rarely been tested empirically. Protection and feeding by parents might accelerate offspring growth by allowing offspring to allocate more resources to growth (resource-allocation hypothesis). Protected young could shift investment away from defensive adaptations toward growth (defensive reallocation), and parental feeding should increase the total amount of assimilated resources (energy intake). Alternatively, rapid growth can be costly due to damage caused by reactive oxygen species, and parental protection might facilitate slower growth to avoid this (costly acceleration hypothesis). We tested these hypotheses along with the suggestion that egg and adult size are correlated with growth in a common-garden study of 17 species of carrion beetles (Silphinae, a subfamily of the beetle family Staphylinidae). Our results were consistent with the resource-allocation hypothesis but did not support the costly acceleration hypothesis or the idea that egg or adult size constrains growth. Species that are normally protected by parents grew faster, not slower, than those that are not. This was true even when their parents were removed and could not feed, supporting the concept of defensive reallocation. As expected based on greater energy intake, the young of species with parental feeding grew faster when their parents were present than when they were not. When phylogeny was accounted for, neither egg nor adult size was related to early growth rate.